Many organic liquids can be mixed with water, in many cases to an unlimited extent. Dehydration purely by distilling requires a large amount of energy. Other problems then arise if the organic liquid forms an azeotropic mixture with water. Some examples of these are ethanol/water, isopropanol/water, ethylacetate/water and pyridine/water. An entrainer is generally used in such cases. The classic example is the use of benzene as an entrainer in dehydrating ethanol. Some of the entrainer is necessarily lost with the exhaust air and waste water. Apart from the resulting costs, the loss of the entrainer puts a strain on the environment. A search has therefore been made for other methods of dehydrating mixtures of organic liquids and water. In principle, the use of membrane permeation for dehydration has been known for a long time; however, no methods which could be used in practice have so far been available.